38 



CHORDATE ORGANIZATION 



The afferent fibres of the dorsal roots are unique among chordates 

 in that the cell bodies are not collected into spinal ganglia but mostly 

 lie within the central nervous system. At least three types of central 

 neuron send fibres that terminate as free nerve endings in the skin. In 

 addition, on the head and tail there are peripheral receptor cells, 

 sending fibres centrally, also complicated encapsulated organs in the 

 metapleural folds (Bone, i960). There are numerous large multipolar 

 nerve-cells, presumably afferent, just beneath the atrial epithelium. 



5 cut. 





bucc. ep 



Fig. 14. Sagittal section through the front end of amphioxus. 

 bucc.ep. buccal epithelium; cer. cerebral vesicle, with large nerve-cells; ep. epidermis; 

 my. myotome; n. notochord; p. pigment spot; s.cut. subcutis; vent, ventricle of cerebral vesicle. 



(After Krause.) 



These cells have many branched dendrites and an axon that runs 

 through a dorsal root to the spinal cord. Their status is discussed on 



P-33- 



The spinal cord has only a narrow lumen and its elements are 



arranged as in vertebrates, namely, ependyma close to the canal, cell 

 layer ('grey matter'), and outer fibrous layer ('white matter'). The cells 

 are not arranged clearly in horns as they are in vertebrates. The most 

 conspicuous cells are the giant cells, which lie dorsally in the anterior 

 and posterior parts but are absent from about the 13th to 39th seg- 

 ments. Each of these cells has many dendrites, branching in the region 

 of entry of the dorsal root fibres, and a single axon, which runs back- 

 wards in the front part of the body, forwards in the hind, passing in 

 each case for the whole length of the cord. A median giant fibre, 

 which runs ventrally for the length of the cord, lies close to the 

 viscero-motor cells that probably produce the 'coughing' movements 

 of the atrium (p. 33). 



